Electrically conducting organic polymer/nanoparticle composites and methods for use thereof

ABSTRACT

Compositions are provided comprising aqueous dispersions of electrically conducting organic polymers and a plurality of nanoparticles. Films cast from invention compositions are useful as buffer layers in electroluminescent devices, such as organic light emitting diodes (OLEDs) and electrodes for thin film field effect transistors. Buffer layers containing nanoparticles have a much lower conductivity than buffer layers without nanoparticles. In addition, when incorporated into an electroluminescent (EL) device, buffer layers according to the invention contribute to higher stress life of the EL device.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/669,422, filed Sep. 24, 2003, and claims priority from U.S.Provisional Application No. 60/413,114, filed Sep. 24, 2002.

FIELD OF THE INVENTION

The invention relates to the use of conductive organic polymers in theproduction of pixelated electroluminescent devices, such as organiclight emitting diodes, and thin film field effect transistor electrodes.

BACKGROUND

Conductive organic polymers originally attracted the attention ofresearchers over 20 years ago. The interest generated by these polymerscompared to conventional conducting materials (e.g., metals,semiconductive metal oxides) was largely due to factors such as lightweight, flexibility, durability, and potential ease of processing. Todate the most commercially successful conductive organic polymers arethe polyanilines and polythiophenes, which are marketed under a varietyof tradenames. These materials can be prepared by polymerizing anilineor dioxythiophene monomers in aqueous solution in the presence of awater soluble polymeric acid, such as poly(styrenesulfonic acid) (PSS),as described in, for example, U.S. Pat. No. 5,300,575 entitled“Polythiophene dispersions, their production and their use.” The recentdevelopment of electroluminescent (EL) devices for use in light emissivedisplays and thin film field effect transistors for use as electrodeshas resulted in a new area of interest in conductive organic polymers.EL devices such as organic light emitting diodes (OLEDs) containingconductive organic polymers generally have the following configuration:

-   -   anode/buffer layer/EL polymer/cathode        The anode is typically any material that has the ability to        inject holes into the otherwise filled π-band of the        semiconducting, EL polymer, such as, for example, indium/tin        oxide (ITO). The anode is optionally supported on a glass or        plastic substrate. The EL polymer is typically a conjugated        semiconducting organic polymer such as        poly(paraphenylenevinylene) or polyfluorene. The cathode is        typically any material, such as Ca or Ba, that has the ability        to inject electrons into the otherwise empty π*-band of the        semiconducting, EL polymer.

The buffer layer is typically a conductive organic polymer whichfacilitates the injection of holes from the anode into the EL polymerlayer. The buffer layer can also be called a hole-injection layer, ahole transport layer, or may be characterized as part of a bilayeranode. Typical aqueous-dispersible conductive organic polymers employedas buffer layers are the emeraldine salt form of polyaniline (PAni) or apolymeric dioxyalkylenethiophene doped with a polymeric sulfonic acid.

While the buffer layer must have some electrical conductivity in orderto facilitate charge transfer, the highest conductivity of buffer layerfilms derived from commonly known aqueous polyaniline or polythiophenedispersion is generally in the range of about 10⁻³ S/cm. Theconductivity is about three order magnitude higher than necessary.Indeed, in order to prevent cross-talk between anode lines (or pixels),the electrical conductivity of the buffer layers should be minimized toabout 10⁻⁶ S/cm without negatively affecting the light emittingproperties of a device containing such a buffer layer For example, afilm made from a commercially available aqueouspoly(ethylenedioxythiophene) dispersion, Baytron-P VP AI 4083 from H. C.Starck, GmbH, Leverkusen, Germany, has conductivity of ˜10 ⁻³ S/cm. Thisis too high to avoid cross-talk between pixels. Accordingly, there is aneed for high resistance buffer layers for use in electroluminescentdevices.

SUMMARY OF THE INVENTION

Compositions are provided comprising aqueous dispersions of electricallyconducting organic polymers and a plurality of nanoparticles. Inventioncompositions are capable of providing continuous, smooth thin films asbuffer layers in electroluminescent devices, such as organic lightemitting diodes (OLEDs) or as electrodes for thin film field effecttransistors. Nanoparticles contemplated for use in the practice of theinvention can be inorganic or organic. Buffer layers containinginorganic or organic nanoparticles have a much lower conductivity thanbuffer layers without such nanoparticles. When incorporated into anelectroluminescent (EL) device, buffer layers according to the inventionprovide high resistance while contributing to higher stress life of theEL device.

In accordance with another embodiment of the invention, there areprovided electroluminescent devices comprising buffer layers cast frominvention aqueous dispersions.

In accordance with a further embodiment of the invention, there areprovided methods for reducing the conductivity of an electricallyconductive organic polymer film cast from an aqueous dispersion of anelectrically conducting polymer onto a substrate, comprising adding aplurality of nanoparticles to the aqueous dispersion.

In a still further embodiment of the invention, there are providedmethods for producing buffer layers having increased thickness, themethod comprising adding a plurality of nanoparticles to an aqueousdispersion of a conductive organic polymer, and casting a buffer layerfrom said aqueous dispersion onto a substrate.

In yet another embodiment of the invention, there are provided thin filmfield effect transistor electrodes cast from invention aqueousdispersions.

In a still further embodiment of the invention, there provided methodsfor increasing conductivity of thin film field effect transistorelectrodes cast from aqueous dispersion onto a substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of an electronic device thatincludes a buffer layer according to the invention.

FIG. 2 illustrates a cross-sectional view of a thin film field effecttransistor that includes an electrode according to the invention.

DETAILED DESCRIPTION

Compositions are provided comprising aqueous dispersions of electricallyconducting organic polymers and a plurality of nanoparticles. As usedherein, the term “dispersion” refers to a continuous medium containing asuspension of minute particles. In accordance with the invention, the“continuous medium” is typically an aqueous liquid, e.g., water.Nanoparticles according to the invention can be inorganic or organic. Asused herein, the term “inorganic” means that the nanoparticles aresubstantially free of carbon. As used herein, the term “organic” meansthat the nanoparticles are composed substantially of carbon. As usedherein, the term “nanoparticle” refers to particles having sizeslessthan 1000 nanometers (nm).

Compositions according to the invention typically contain a continuousaqueous phase in which the electrically conducting organic polymer isdispersed. Electrically conductive organic polymers contemplated for usein the practice of the invention include, for example, all forms of thepolyanilines (e.g., leucoemeraldine, emeraldine, nigraniline, and thelike) which are capable of forming acid/base salts to render thepolymers electrically conductive. It is well known that different formsof polyaniline polymers can be synthesized, depending upon the degree ofoxidation. Polyaniline (PAni) can generally be described as beingcomposed of monomer units having aromatic amine nitrogen atoms, as inFormula I below, and/or aromatic imine nitrogen atoms, as in Formula IIbelow:

wherein:

n is an integer from 0 to 4;

and

R is independently selected so as to be the same or different at eachoccurrence and is selected from alkyl, alkenyl, alkoxy, cycloalkyl,cycloalkenyl, alkanoyl, alkylthio, aryloxy, alkylthioalkyl, alkylaryl,arylalkyl, amino, alkylamino, dialkylamino, aryl, alkylsulfinyl,alkoxyalkyl, alkylsulfonyl, arylthio, arylsulfinyl, alkoxycarbonyl,arylsulfonyl, carboxylic acid, halogen, cyano, or alkyl substituted withone or more of sulfonic acid, carboxylic acid, halo, nitro, cyano orepoxy moieties; or any two R groups together may form an alkylene oralkenylene chain completing a 3, 4, 5, 6, or 7-membered aromatic oralicyclic ring, which ring may optionally include one or more divalentnitrogen, sulfur or oxygen atoms.

Although formulae I and II show the monomer units in the unprotonatedform, it is known that in the presence of an acid (such as,poly(2-acrylamido-2-methyl-1-propanesulfonic acid (PAAMPSA),poly(styrenesulfonic acid) (PSS), and the like), the basic nitrogenatoms will be protonated to form a salt. The relative proportion ofimine nitrogen atoms to amine nitrogen atoms increases with increasingoxidation. In one embodiment, the polyaniline is the emeraldine baseform in which the ratio of monomer units having Formula I to thosehaving Formula II is 2:1. In this embodiment, the ratio of aminenitrogen atoms to imine nitrogen atoms is 1:1.

In another embodiment, the electrically conductive organic polymer ispoly(dioxythiophene). Poly(dioxythiophenes) contemplated for use in thepractice of the invention have Formula III:

wherein:

-   -   R₁ and R₁′ are each independently selected from hydrogen and        alkyl having 1 to 4 carbon atoms,    -   or R₁ and R₁′ taken together form an alkylene chain having 1 to        4 carbon atoms, which may optionally be substituted by alkyl or        aromatic groups having 1 to 12 carbon atoms, or a        1,2-cyclohexylene radical, and    -   n is greater than about 9.

The electrically conductive organic polymers used in the inventioncompositions and methods are typically prepared by oxidativelypolymerizing the corresponding monomers in aqueous solution containing apolymeric acid (e.g., PAAMPSA, PSS, and the like). The oxidativepolymerization is carried out using an oxidizing agent such as ammoniumpersulfate, sodium persulfate, or the like. Thus, for example, whenaniline is oxidatively polymerized in the presence of PAAMPSA, theelectrically conductive acid/base salt PAni/PAAMPSA is formed. Whenethylenedioxythiophene (EDT) is oxidatively polymerized in the presenceof PSS, the electrically conductive acid/base saltpoly(ethylenedioxythiophene) (PEDT)/PSS is formed.

The aqueous solution also can include a polymerization catalyst such asferric sulfate, ferric chloride, and the like. The polymerization istypically carried out at low temperatures, e.g., between −10° C. and 30°C. After completion of the polymerization reaction, the polymers areoptionally isolated by precipitation from aqueous dispersion using anon-solvent for the polymers, e.g., acetone, and the like. When theelectrically conductive organic polymer is isolated, the material istypically refined to produce polymer particles having a size less thanabout 1000 nm. In one embodiment, the polymer particles are less thanabout 500 nm. In another embodiment, the polymer particles are less thanabout 50 nm. The isolated electrically conductive organic polymerparticles are then either directly combined with an aqueous dispersionof nanoparticles or the conductive organic polymer particles areredispersed in water prior to combination with an aqueous dispersion ofnanoparticles.

In another embodiment of the invention methods, the oxidativepolymerization is carried out in the presence of nanoparticles, therebyproducing an aqueous dispersion without isolating the electricallyconductive organic polymer. For example, nanoparticles may be added toan aqueous solution containing aniline monomers, thereby forming adispersion. An oxidizing agent can then be added to polymerize themonomers in the presence of the nanoparticles. This embodiment of theinvention is economically attractive since it provides invention aqueousdispersions in a “one-pot” synthesis. Invention aqueous dispersionsprepared by either method provide the advantage of being easilyfiltered, for example, through a Millex 0.45 μm HV filter. Thus,invention aqueous dispersions readily provide continuous, smooth films.

Organic additives, such as steric stabilizers, may optionally be addedto the aqueous solution prior to oxidative polymerization. Theseadditives facilitate formation of electrically conductive organicpolymers having nanometer sized particles. Organic additives include,for example, polyacrylamide, polyvinylalcohol, poly(2-vinylpyridine),poly(vinyl acetate), poly(vinyl methyl ether), poly(vinylpyrrolidone),poly(vinyl butyral), and the like.

Nanoparticles contemplated for use in the practice of the presentinvention can be either inorganic or organic. Inorganic nanoparticlescontemplated for use in the practice of the invention include alumina,silica, metallic nanoparticles, electrically semiconductive metaloxides, and the like. In one embodiment, the electrically semiconductivemetal oxide is selected from mixed valence metal oxides, such as zincantimonates, and the like. In another embodiment, the metallicnanoparticles are molybdenum nanoparticles. Organic nanoparticlescontemplated for use in the practice of the invention include colloidalsulfonic acids (such as perfluoroethylene sulfonates, and the like),polyacrylates, polyphosphonates, carbon nanotubes, and the like.

Nanoparticles contemplated for use in the practice of the invention mayhave a variety of shapes and sizes. In one embodiment, the nanoparticlesare substantially spherical. In another embodiment, the nanoparticlesare substantially elongated such as metal nanowires. Nanoparticlescontemplated for use in the practice of the invention typically have anaverage particle diameter less than about 500 nm. In another embodiment,the nanoparticles have an average particle diameter less than about 100nm. In still another embodiment, the nanoparticles have an averageparticle diameter less than about 50 nm. In another embodiment, theaspect ratio of elongated nano-particles is greater than 1 to 100.Aspect ratio is defined as ratio of particle width to particle length.For elongated particles, the “particle size” is considered to be theparticle width. In another embodiment, the nano-particles have anirregular geometry. For irregularly-shaped particles, the “particlesize” is considered to be size of the smallest screen opening throughwhich the particle will pass. In another embodiment of the invention,there are provided buffer layers cast from aqueous dispersionscomprising electrically conductive organic polymers and nanoparticles.Both the electrically conducting polymers and the nanoparticles can bereadily dispersed in water. Thus, continuous, smooth films can beproduced by casting from aqueous dispersions containing electricallyconducting polymers and nanoparticles. Invention buffer layers have areduced conductivity relative to buffer layers of identical compositionexcept the inorganic nanoparticles are absent. Electrical resistivity isinversely proportional to electrical conductivity. Thus, as employedherein, the phrases “high resistance” and “low conductivity” are usedinterchangeably with reference to the buffer layers described herein. Asused herein, the phrases “high resistance” and “low conductivity” eachrefer to a conductivity level less than that of a commercially availablebuffer layers, i.e., less than about 1.0×10⁻³ S/cm. In anotherembodiment, the resistivitys preferably less than 1.0×10⁻⁵ S/cm.Resistivity and conductivity values are typically reported in units ofohm-centimeter (ohm-cm) and Siemens per centimeter (S/cm), respectively.As used herein, conductivity values are reported (using the unit S/cm)rather than resistivity values.

In another embodiment of the invention, there are provided electronicdevices comprising at least one electroluminescent layer (usually asemiconductor conjugated polymer) positioned between two electricalcontact layers, wherein at least one of the layers of the deviceincludes a buffer layer of the invention. As shown in FIG. 1, a typicaldevice has an anode layer 110, a buffer layer 120, an electroluminescentlayer 130, and a cathode layer 150. Adjacent to the cathode layer 150 isan optional electron-injection/transport layer 140. Between the bufferlayer 120 and the cathode layer 150 (or optional electroninjection/transport layer 140) is the electroluminescent layer 130.

The device may include a support or substrate (not shown) that can beadjacent to the anode layer 110 or the cathode layer 150. Mostfrequently, the support is adjacent the anode layer 110. The support canbe flexible or rigid, organic or inorganic. Generally, glass or flexibleorganic films are used as a support. The anode layer 110 is an electrodethat is more efficient for injecting holes than the cathode layer 150.The anode can include materials containing a metal, mixed metal, alloy,metal oxide or mixed oxide. Suitable materials include the mixed oxidesof the Group 2 elements (i.e., Be, Mg, Ca, Sr, Ba, Ra), the Group 11elements, the elements in Groups 4, 5, and 6, and the Group 8-10transition elements. If the anode layer 110 is to be light transmitting,mixed oxides of Groups 12, 13 and 14 elements, such as indium-tin-oxide,may be used. As used herein, the phrase “mixed oxide” refers to oxideshaving two or more different cations selected from the Group 2 elementsor the Groups 12, 13, or 14 elements. Some non-limiting, specificexamples of materials for anode layer 110 include indium-tin-oxide(“ITO”), aluminum-tin-oxide, gold, silver, copper, and nickel. The anodemay also comprise an organic material such as polyaniline orpolythiophene.

The anode layer 110 may be formed by a chemical or physical vapordeposition process or spin-cast process. Chemical vapor deposition maybe performed as a plasma-enhanced chemical vapor deposition (“PECVD”) ormetal organic chemical vapor deposition (“MOCVD”). Physical vapordeposition can include all forms of sputtering, including ion beamsputtering, as well as e-beam evaporation and resistance evaporation.Specific forms of physical vapor deposition include rf magnetronsputtering and inductively-coupled plasma physical vapor deposition(“IMP-PVD”). These deposition techniques are well known within thesemiconductor fabrication arts.

Usually, the anode layer 110 is patterned during a lithographicoperation. The pattern may vary as desired. The layers can be formed ina pattern by, for example, positioning a patterned mask or resist on thefirst flexible composite barrier structure prior to applying the firstelectrical contact layer material. Alternatively, the layers can beapplied as an overall layer (also called blanket deposit) andsubsequently patterned using, for example, a patterned resist layer andwet chemical or dry etching techniques. Other processes for patterningthat are well known in the art can also be used. When the electronicdevices are located within an array, the anode layer 110 typically isformed into substantially parallel strips having lengths that extend insubstantially the same direction.

The buffer layer 120 is usually cast onto substrates using a variety oftechniques well-known to those skilled in the art. Typical castingtechniques include, for example, solution casting, drop casting, curtaincasting, spin-coating, screen printing, inkjet printing, and the like.Alternatively, the buffer layer can be patterned using a number of suchprocesses, such as ink jet printing.

The electroluminescent (EL) layer 130 may typically be a conjugatedpolymer such as poly(paraphenylenevinylene) or polyfluorene. Theparticular material chosen may depend on the specific application,potentials used during operation, or other factors. The EL layer 130containing the electroluminescent organic material can be applied fromsolution by any conventional technique, including spin-coating, casting,and printing. The EL organic materials can be applied directly by vapordeposition processes, depending upon the nature of the materials. Inanother embodiment, an EL polymer precursor can be applied and thenconverted to the polymer, typically by application of heat or othersource of external energy (e.g., visible light or UV radiation).

Optional layer 140 can function both to facilitate electroninjection/transport, and also serve as a confinement layer to preventquenching reactions at layer interfaces. More specifically, layer 140may promote electron mobility and reduce the likelihood of a quenchingreaction if layers 130 and 150 would otherwise be in direct contact.Examples of materials for optional layer 140 include metal-chelatedoxinoid compounds (e.g., Alq₃ or the like); phenanthroline-basedcompounds (e.g., 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (“DDPA”),4,7-diphenyl-1,10-phenanthroline (“DPA”), or the like); azole compounds(e.g., 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (“PBD” orthe like), 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole(“TAZ” or the like); other similar compounds; or any one or morecombinations thereof. Alternatively, optional layer 140 may be inorganicand comprise BaO, LiF, Li₂O, or the like.

The cathode layer 150 is an electrode that is particularly efficient forinjecting electrons or negative charge carriers. The cathode layer 150can be any metal or nonmetal having a lower work function than the firstelectrical contact layer (in this case, the anode layer 110). As usedherein, the term “lower work function” is intended to mean a materialhaving a work function no greater than about 4.4 eV. As used herein,“higher work function” is intended to mean a material having a workfunction of at least approximately 4.4 eV.

Materials for the cathode layer can be selected from alkali metals ofGroup 1 (e.g., Li, Na, K, Rb, Cs), the Group 2 metals (e.g., Mg, Ca, Ba,or the like), the Group 12 metals, the lanthanides (e.g., Ce, Sm, Eu, orthe like), and the actinides (e.g., Th, U, or the like). Materials suchas aluminum, indium, yttrium, and combinations thereof, may also beused. Specific non-limiting examples of materials for the cathode layer150 include barium, lithium, cerium, cesium, europium, rubidium,yttrium, magnesium, and samarium.

The cathode layer 150 is usually formed by a chemical or physical vapordeposition process. In general, the cathode layer will be patterned, asdiscussed above in reference to the anode layer 110. If the device lieswithin an array, the cathode layer 150 may be patterned intosubstantially parallel strips, where the lengths of the cathode layerstrips extend in substantially the same direction and substantiallyperpendicular to the lengths of the anode layer strips. Electronicelements called pixels are formed at the cross points (where an anodelayer strip intersects a cathode layer strip when the array is seen froma plan or top view).

In other embodiments, additional layer(s) may be present within organicelectronic devices. For example, a layer (not shown) between the bufferlayer 120 and the EL layer 130 may facilitate positive charge transport,band-gap matching of the layers, function as a protective layer, or thelike. Similarly, additional layers (not shown) between the EL layer 130and the cathode layer 150 may facilitate negative charge transport,band-gap matching between the layers, function as a protective layer, orthe like. Layers that are known in the art can be used. In addition, anyof the above-described layers can be made of two or more layers.Alternatively, some or all of inorganic anode layer 110, the bufferlayer 120, the EL layer 130, and cathode layer 150, may be surfacetreated to increase charge carrier transport efficiency. The choice ofmaterials for each of the component layers may be determined bybalancing the goals of providing a device with high device efficiencywith the cost of manufacturing, manufacturing complexities, orpotentially other factors.

Depending upon the intended application of the electronic device, the ELlayer 130 can be a light-emitting layer that is activated by signal(such as in a light-emitting diode) or a layer of material that respondsto radiant energy and generates a signal, with or without an appliedpotential (such as detectors or voltaic cells). Examples of electronicdevices that may respond to radiant energy are selected fromphotoconductive cells, photoresistors, photoswitches, phototransistors,and phototubes, and photovoltaic cells. After reading thisspecification, skilled artisans will be capable of selecting material(s)that are suitable for their particular applications. The light-emittingmaterials may be dispersed in a matrix of another material, with orwithout additives, but preferably form a layer alone. The EL layer 130generally has a thickness in the range of approximately 50-500 nm.

In organic light emitting diodes (OLEDs), electrons and holes, injectedfrom the cathode 150 and anode 110 layers, respectively, into the ELlayer 130, form negative and positively charged polarons in the polymer.These polarons migrate under the influence of the applied electricfield, forming a polaron exciton with an oppositely charged species andsubsequently undergoing radiative recombination. A sufficient potentialdifference between the anode and cathode, usually less thanapproximately 12 volts, and in many instances no greater thanapproximately 5 volts, may be applied to the device. The actualpotential difference may depend on the use of the device in a largerelectronic component. In many embodiments, the anode layer 110 is biasedto a positive voltage and the cathode layer 150 is at substantiallyground potential or zero volts during the operation of the electronicdevice. A battery or other power source(s) may be electrically connectedto the electronic device as part of a circuit but is not illustrated inFIG. 1.

The invention also provides thin film field effect transistorelectrodes. In thin film field effect transistors, a dielectric polymeror dielectric oxide thin film is provided with a gate electrode on oneside and drain and source electrodes on the other side. Between thedrain and source electrode, an organic semiconducting film is deposited.The organic semiconducting polymer film is typically cast from anorganic solution using aromatic solvent such as toluene, or chlorinatedorganic solvent such as chloroform. To be useful for the electrodeapplication, the conducting polymers and the liquids for dispersing ordissolving the conducting polymers must be compatible with thesemiconducting polymers and the solvents for the semiconducting polymersto avoid re-dissolution of either conducting polymers or semiconductingpolymers. Thin film field effect transistor electrodes fabricated fromconducting polymers should have a conductivity greater than 10 S/cm.However, electrically conducting polymers made with a polymeric acidonly provide conductivity in the range of ˜10 ⁻³ S/cm or lower. In orderto enhance conductivity of the electrically conducting polymers withoutcompromising processability (such as casting, spin-coating, and thelike) a highly conductive additive is needed. Accordingly, in anotherembodiment of the invention, there are provided thin film field effecttransistor electrodes. Invention electrodes are cast from aqueousdispersions containing electrically conducting polymers andnanoparticles. In this embodiment, the nanoparticles are typicallycarbon nanotubes, metal nanoparticles, or metal nanowires resulting inelectrodes having a conductivity of greater than about 10 S/cm. In afurther embodiment, the electrically conducting polymers arepolyanilines/polymeric acid, polydioxyalkylenethiophenes/polymeric acid,or the like.

The invention also provides thin film field effect transistorscontaining invention electrodes. Thin film field effect transistors, asshown in FIG. 2, are typically fabricated in the following manner. Adielectric polymer or dielectric oxide thin film 210 has a gateelectrode 220 on one side and drain and source electrodes, 230 and 240,respectively, on the other side. Between the drain and source electrode,an organic semiconducting film 250 is deposited. Invention aqueousdispersions containing nanowires or carbon nanotubes are ideal for theapplications of gate, drain and source electrodes because of theircompatibility with organic based dielectric polymers and semiconductingpolymers in solution thin film deposition.

In accordance with another embodiment of the invention, there areprovided methods for reducing conductivity of an electrically conductiveorganic polymer film cast from aqueous dispersion onto a substrate to avalue less than about 1×10⁻⁵ S/cm. Such a method can be performed, forexample, by adding a plurality of nanoparticles to the aqueousdispersion of electrically conducting polymers. Surprisingly, it hasbeen discovered that even electrically semi-conductive inorganicnanoparticles, when incorporated into an electrically conductive organicpolymer film as described herein, reduce the conductivity of the polymerfilm. In one embodiment, the electrically conductive organic polymerfilm can be used as a buffer layer in electroluminescent devices. Inanother embodiment, the electrically conductive polymer film isPAni/PAAMPSA.

In a still further embodiment of the invention, there are providedmethods for producing buffer layers having increased thickness. Such amethod can be performed, for example, by adding a plurality ofnanoparticles to an aqueous dispersion of an electrically conductiveorganic polymer, and casting a buffer layer from said aqueous dispersiononto a substrate. Addition of nanoparticles to aqueous dispersions ofconductive polymers produces aqueous dispersions having an increasedviscosity. This enhanced viscosity provides increased control of thethickness of layers cast from the aqueous solutions. Control of bufferlayer thickness is desirable since the appropriate thickness of aproperly functioning buffer layer depends to some extent on the surfaceroughness of the metallic conductive layer onto which the buffer layeris deposited.

The invention will now be described in greater detail by reference tothe following non-limiting examples.

EXAMPLES Comparative Example 1

This comparative example illustrates preparation of PAni/PAAMPSA and itselectrical conductivity and light emitting device properties as a bufferlayer.

Synthesis of PAni/PAAMPSA was carried out in the following manner.Aniline (4.08 g) was dissolved in a 150 ml aqueous solution containingPAAMPSA (6.35 g, PAAMPSA was obtained from Sigma-Aldrich Corp., St.Louis, Mo., USA) in the form of 15 weight % aqueous solution; weightaverage molecular weight of the PAAMPSA is 2,000,000). The aqueoussolution containing aniline and PAAMPSA was placed into a 4-necked 500ml round bottomed flask and cooled to ˜4° C. with an ice/water bath. Thesolution was stirred continuously with a magnetic stirrer. To thechilled aqueous aniline/PAAMPSA solution, a 100 ml aqueous solutioncontaining 4.235 g PAAMPSA and 2.36 g ammonium persulfate was added at aconstant rate over a one hour period. The polymerization was allowed toproceed for 198 minutes.

The reaction mixture was then poured into two centrifuge bottles andcentrifuged at 8000 rpm at 15° C. for 30 minutes. After centrifugation,the supernatant was slowly added to acetone to precipitate the polymericproduct. After precipitation, the solid polymer was washed repeatedly inacetone, filtered, and dried overnight in a vacuum oven (˜18 Hg., N₂bleed, ambient temperature) Yield is 7.03 g.

PAni/PAAMPSA (1.21 g) as prepared above was dispersed in 39.12 gdeionized water, which constitutes 3.0 wt % solid in the aqueoussolution. The dispersion was tested for electrical conductivity andlight emission properties. Commercially available indium tin oxide(ITO)/glass plates having ITO layers 100 to 150 nm thick were cut intosamples 30 mm×30 mm in size. The ITO layers were subsequently etchedwith oxygen plasma. The ITO on the glass substrates to be used forelectrical conductivity measurements were etched into a series ofparallel lines of ITO to be used as electrodes. The ITO on thesubstrates to be made into LEDs for light emission measurement wereetched into a 15 mm×20 mm layer of ITO to serve as the anode. Theaqueous dispersion was then spin-coated onto each of the ITO/glasssubstrates at a spinning speed of 1,200 rpm. The resulting PAni/PAAMPSAlayer was about 137 nm thick.

A sample for viscosity measurement was prepared as follows: 0.0989 g ofthe PAni/PAAMPSA was mixed with 9.9081 g deionized water, whichconstitutes 0.99 wt % PAni/PAAMPSA in the dispersion. The mixture wasmagnetically stirred overnight. It should be noted that the coatingthickness is lower (137 nm vs. 300 nm) when compared with InventionExample 4 in spite of lower spin coating speed (1,200 rpm vs. 6,000rpm). The comparison shows that the dispersion has a lower viscositythan the dispersion in Invention Example 4.

The PAni/PAAMPSA coated ITO/glass substrates were dried in nitrogen at90° C. for 30 minutes before measuring for electrical conductivity.Electrical conductivity was determined by measuring the resistancebetween two electrodes and was calculated to be 3.6×10⁻⁵ S/cm based onthe resistance, the thickness of the conductive layer and the distancebetween the two electrodes used to measure the resistance. Theconductivity is shown in Table I.

For light emission measurements, when incorporated into a light emittingdiode (LED), the PAni/PAAMPSA layer was top-coated with a super-yellowemitter poly(substituted-phenylene vinylene) (PDY 131 obtained fromCovion Company, Frankfurt, Germany) to serve as the activeelectroluminescent (EL) layer. The thickness of the EL layer wasapproximately 70 nm. All film thicknesses were measured with a TENCOR500 Surface Profiler. For the cathode, Ba and Al layers were vapordeposited onto the EL layer at a pressure of 1.3×10⁻⁴ Pa. The finalthickness of the Ba layer was 3 nm; the thickness of the Al layer on topof the Ba layer was 300 nm. LED device performance was tested asfollows. Measurements of current vs. applied voltage, light emissionbrightness vs. applied voltage, and light emission efficiency(candela/ampere-cd/A) were measured with a Keithley 236 source-measureunit from Keithley Instrument Inc. (Cleveland, Ohio), and a S370Optometer with a calibrated silicon photodiode (UDT Sensor, Inc.,Hawthorne, Calif.). Current was applied to each of five LED with acurrent density of 8.3 mA/cm² at room temperature. The average voltageto achieve the current density was 4.2 volts and the average lightefficiency, and light emission brightness were 8.3 cd/A and 115 cd/m²,respectively, as summarized in Table I. Table I also shows that devicehalf-life at 80° C. and 3.3 mA/cm² current density was 12 hrs.

Invention Example 1

This example illustrates preparation of an aqueous PAni/PAAMPSAdispersion with silica nanoparticles and its electrical conductivity andlight emission properties as a buffer layer.

PAni/PAAMPSA (0.63 g) prepared as in Comparative Example 1 was combinedwith 0.75 g Snowtex-UP® (0.75 g, Nissan Chemical Industries, Ltd. Tokyo,Japan), which contains 0.152 g silica nanoparticles, and 24.07 gdeionized water. Snowtex-UP® is an aqueous dispersion having a pH from9.0 to 10.5, and having a silica particle size of about 5-20 nm in widthand about 40-300 nm in length. The silica:PAni/PAAMPSA weight ratio is4.1:1.

The dispersion was tested for electrical conductivity and light emissionproperties in the same manner as described in Comparative Example 1. Asshown in the results summarized in Table 1, a buffer layer cast from thedispersion of Invention Example 1 has a lower conductivity (8.2×10⁻⁷S/cm vs. 3.6×10⁻⁵ S/cm) and higher half-life (29 hours vs. 12 hours)when compared with Comparative Example 1. This example demonstrateseffect of nano-particles on reducing conductivity with enhanced devicelife-time.

Invention Example 2

This example illustrates preparation of an invention aqueousPAni/PAAMPSA dispersion with colloidal silica and its electricalconductivity and light emission properties as a buffer layer.

PAni/PAAMPSA (0.61 g) as prepared in Comparative Example 1 was combinedwith Snowtex-O® (0.75 g, from Nissan Chemical Industries, Ltd. Tokyo,Japan), which contains 0.153 g silica nanoparticles, and 23.47 gdeionized water. Snowtex-O® is an aqueous dispersion having a pH rangeof 2-4 and having a silica particle size of 10-20 nm. Thesilica:PAni/PAAMPSA weight ratio is 3.99:1

The dispersion was tested for electrical conductivity and light emissionproperties in the same manner as described in Comparative Example 1. Asshown in the results summarized in Table 1, a buffer layer cast from thedispersion of Invention Example 2 has a lower conductivity (7.6×10⁻⁷S/cm vs. 3.6×10⁻⁵ S/cm) and higher half-life (30 hours vs. 12 hours)when compared with Comparative Example 1. This example againdemonstrates effect of nano-particles on reducing conductivity withenhanced device life-time.

Invention Example 3

This example illustrates preparation of an invention aqueousPAni/PAAMPSA dispersion with electrically semi-conductive oxide and itselectrical conductivity and light emission properties as a buffer layer.

PAni/PAAMPSA (0.90 g) as prepared in Comparative Example 1 was combinedwith Celnax CX-Z300H® (1.97 g, a zinc antimonite from Nissan ChemicalIndustries, Ltd. Tokyo, Japan), which contains 0.601 g conductive oxideparticles, and 48.53 g deionized water. The Celnax CX-Z300H® is anaqueous dispersion having a pH of 6.85, and having 20 nm oxidenanoparticles. Conductivity of the oxide powder is 3.6 S/cm measured ona compressed dried pellet at a pressure of 160 Kg/cm². Theoxides:PAni/PAAMPSA weight ratio is 1.50:1

The dispersion was tested for electrical conductivity and light emissionproperties in the same manner as described in Comparative Example 1. Asshown in the results summarized in Table 1, a buffer layer cast from thedispersion of Invention Example 3 has a lower conductivity (8.3×10⁻⁸S/cm vs. 3.6×10⁻⁵ S/cm) and higher half-life (61 hours vs. 12 hours)when compared with Comparative Example 1. This example againdemonstrates effect of nano-particles on reducing conductivity withenhanced device life-time.

Invention Example 4

This example illustrates the preparation of an invention aqueousdispersion of PAni/PAAMPSA in the presence of SiO₂ nanoparticles and itselectrical conductivity and light emitting properties as a buffer layer.

Synthesis of PAni/PAAMPSA in the presence of SiO₂ nanoparticles wascarried out in the following manner. PAAMPSA (36.32 g of 15 wt % aqueoussolution from Sigma-Aldrich Corp., St. Louis, Mo., USA) was placed in a250 Nalgene® plastic bottle. To the PAAMPSA solution was addedSnowtex-UP® (34.33 g from Nissan Chemical Industries, Ltd. Tokyo,Japan). Snowtex-UP® is an aqueous dispersion with pH of 9.0 to 10.5,containing silica particles sized 5-20 nm in width and 40-300 nm inlength. The PAAMPSA/Snowtex-Up® silica mixture was dispersed indeionized water (150 ml). To this dispersion was added aniline (4.08 g).The aqueous PAAMPSA/Snowtex-Up®/aniline mixture was placed in a 4-necked500 ml Round Bottomed Flask and then cooled to ˜4° C. with an ice/watermixture. The solution was stirred continuously with a magnetic stirrer.To the chilled PAAMPSA/Snowtex-Up®/aniline dispersion 100 ml aqueoussolution containing 4.493 g PAAMPSA (as above) and ammonium persulfate(2.33 g) was added over a one hour period. The reaction was allowed toproceed for 180 minutes.

The reaction mixture was then poured into two centrifuge bottles andcentrifuged at 8000 rpm at 15° C. for 30 minutes. After centrifugation,the supernatant was slowly added to acetone to precipitate the polymericproduct. After precipitation, the solid polymer was washed repeatedly inacetone, filtered, and dried overnight in a vacuum oven (˜18 Hg., N₂bleed, ambient temperature) (Yield was 14.19 g). It should be noted thatthe yield is almost twice the yield in Comparative Example 1. Theincrease in yield indicates that SiO₂ nanoparticles are present withinthe PAni/PAAMPSA.

PAni/PAAMPSA/SiO₂ (1.20 g, as prepared above) was dispersed in 38.80 gdeionized water, which dispersion constitutes 3.0 wt % solid in thewater. A buffer layer was cast onto an ITO substrate as in the previousexamples. For light emission measurements, the PAni/PAAMPSA/silica layerwas then top-coated with a super-yellow emitterpoly(substituted-phenylene vinylene) (PDY 131 obtained from CovionCompany, Frankfurt, Germany) to serve as the active electroluminescent(EL) layer in an LED device. The thickness of the EL layer wasapproximately 70 nm. Thickness of all films was measured with a TENCOR500 Surface Profiler. For the cathode, Ba and Al layers were vapordeposited on top of the EL layer under a vacuum of 1.3×10⁻⁴ Pa. Thefinal thickness of the Ba layer was 3 nm; the thickness of the Al layeron top of the Ba layer was 300 nm. LED device performance was tested asfollows. Measurements of current vs. applied voltage, light emissionbrightness vs. applied voltage, and light emission efficiency(candela/ampere-cd/A) were measured with a Keithley 236 source-measureunit from Keithley Instrument Inc. (Cleveland, Ohio), and a S370Optometer with a calibrated silicon photodiode (UDT Sensor, Inc.,Hawthorne, Calif.). Current was applied to each of five LED devices witha current density of 8.3 mA/cm² at room temperature. The average voltageto achieve the current density was 4.3 volts and the average lightefficiency, and light emission brightness were 5.3 cd/A and 130 cd/m²,respectively, as summarized in Table I. Table I also shows that devicehalf-life at 80° C. and 3.3 mA/cm² current density was 42 hrs. It shouldbe noted that the half-life is enhanced 4× and emission intensity ishigher (130 cd/m² vs. 115) when compared with the PAni/PAAMPSA withoutsilica in spite of the thickness being 2.2 times the thicknessillustrated in Comparative Example 1.

TABLE 1 Effect of inorganic nanoparticles added to PAni/PAAMPSA on lightemission properties of OLEDs on a glass substrate Brightness Half-lifeCoating Voltage Efficiency (Cd/m²) @ (hr) @ 3.3 thickness Conductivity(volt) @ 8.3 (Cd/A) @ 8.3 3.3 mA/cm², mA/cm², Example (nm) (S/cm) mA/cm²mA/cm² 80° C. 80° C. Comparative 137 @ 3.6 × 10⁻⁵ 4.2 8.3 115 12 Example1 1,200 rpm Invention 114 @ 8.2 × 10⁻⁷ 4.8 8.1 135 29 Example 1 1,200rpm Invention 166 @ 7.6 × 10⁻⁷ 4.9 7.4 108 30 Example 2 1,200 rpmInvention 113 @ 8.3 × 10⁻⁸ 4.1 8.0 148 61 Example 3 1,200 rpm Invention300 @ 9.0 × 10⁻⁷ 4.3 5.3 130 42 Example 4 6,000 rpm

As shown in Table 2, the aqueous dispersion of Invention Example 4produces a 300 nm coating thickness at a spinning speed of 6,000 rpm.The coating thickness is much higher than that of Comparative Example 1(300 nm vs. 137 nm) despite a higher spin-coating speed (6,000 rpm vs.1,200 rpm). The comparison shows that the PAni/PAAMPSA polymerized withsilica nanoparticles has a higher viscosity than PAni/PAAMPSApolymerized without silica nanoparticles. As shown in Table II, thisincreased viscosity produces buffer layers having an increasedthickness. The dispersion for viscosity measurement was prepared asfollows: 0.0999 g of the PAni/PAAMPSA/silica was mixed with 9.9164 gdeionized water, which constitutes 1.00 wt % PAni/PAAMPSA/silica in thedispersion.

TABLE 2 Viscosity of Aqueous PAni/PAAMPSA Dispersions Viscosity (cps)Sample 10 s⁻¹ 100 s⁻¹ 1000 s⁻¹ Comparative 5.50 4.52 4.10 Example 1Invention 8.30 6.80 6.15 Example 4

Invention Example 5

This example demonstrates the integrity of solid films, prepared from acommercial aqueous PEDT dispersion, in organic solvents

In this example, a commercially available aqueous PEDT dispersion(Baytron-P VP AI 4083 from H. C. Starck, GmbH, Leverkusen, Germany),which has conductivity of ˜10 ⁻³ S/cm, was dried to solid films in aglass beaker under a nitrogen flow at room temperature. The dried filmswere mixed with common organic solvents (such as toluene, chloroform,dichloromethane, or the like) used to dissolve organic semiconductingpolymers in the manufacture of thin film field effect transistors. Thefilm flakes were not swollen by either of the organic liquids nordiscolored the liquid. This result clearly demonstrates that PEDT filmsmade from the Baytron-P are compatible with the organic solvents forsemiconducting polymers, thereby demonstrating utility as electrodes forthin film field effect transistors.

Invention Example 6

This example demonstrates the integrity of solid films, prepared from anaqueous dispersion of PAni/PAAMPSA in organic solvents.

In this example, the aqueous dispersion of PAniPAAMPSA prepared inComparative Example 1 was dried to solid films in a glass beaker under anitrogen flow at room temperature. The dried films were mixed withcommon organic solvents (such as toluene, chloroform, dichloromethane,or the like) used to dissolve organic semiconducting polymers in themanufacture of thin film field effect transistors. The film flakes werenot swollen by either of the organic liquids nor discolored the liquid.This result clearly demonstrated that the PAni/PAAMPSA films made fromaqueous dispersions of PAni/PAAMPSA are compatible with the organicsolvents for semiconducting polymers, thereby demonstrating utility aselectrodes for thin film field effect transistors.

Invention Example 7

This example illustrates the preparation of an invention aqueousdispersion containing a polyaniline/polymeric acid or apoly(dioxyethylenethiophene)/polymeric acid and highly conductivenanoparticles.

As shown in Comparative Example 1, electrical conductivity ofPAni/PAAMPSA cast from an aqueous dispersion of PAni/PAAMPSA is only3.6×10⁻⁵ S/cm, which is not sufficient for the application as gate,drain or source electrodes of thin film field effect transistors. Thehighest conductivity ever achieved from aqueous dispersion of PAni, forexample from Ormecon (Ammersbeck, Germany) or PEDT, for exampleBaytron-P, is about 10⁻³ S/cm, which is still too low for theapplication. However, use of invention aqueous conducting polymerdispersions containing nanoparticles such as nano-wire or nano-particlesof metal or carbon nanotubes dramatically increases the conductivity ofelectrodes cast from these aqueous dispersions. For example, metallicmolybdenum wires having diameter of 15 nm and conductivity of 1.7×10⁴S/cm can be used for enhanced conductivity. Carbon nanotubes havingdimension of 8 nm diameter and 20 μm length and conductivity of 60 S/cmcan also be used to enhance conductivity. Because of the highconductivity of the nanoparticles and dispersibility of the particles inwater, composite aqueous dispersions consisting of the electricallyconducting polymers and the highly conductive nanoparticles can bereadily prepared for fabricating continuous, smooth films as drain,source or gate electrodes in thin film field effect transistors.

While the invention has been described in detail with reference tocertain preferred embodiments thereof, it will be understood thatmodifications and variations are within the spirit and scope of thatwhich is described and claimed.

What is claimed is:
 1. A composition comprising an aqueous dispersion ofan electrically conductive organic polymer and a plurality ofnanoparticles wherein said electrically conductive organic polymer isselected from polyaniline withpoly(2-acrylamido-2-methyl-1-propanesulfonic acid) as the counterion(Pani/PAAMPSA) and poly(ethylenedioxythiophene) withpoly(styrenesulfonic acid) as the counterion (PEDT/PSS) and whereinnanoparticles are selected from the group consisting of inorganicnanoparticles, and wherein the composition is capable of forming abuffer layer having a conductivity of less than 1×10⁻⁵ S/cm.
 2. Acomposition according to claim 1, wherein said inorganic nanoparticlesare selected from silica, alumina and electrically conductive metaloxides.
 3. A composition according to claim 1, wherein saidnanoparticles have a particle size less than about 500 nm.
 4. Acomposition according to claim 1, wherein said nanoparticles have aparticle size less than about 250 nm.
 5. A composition according toclaim 1, wherein said nanoparticles have a particle size less than about50 nm.
 6. A composition according to claim 2, wherein the weight ratioof silica:electrically conductive polymer is about 4:1.